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Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
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Inventories [Abstract] |  |  |
Fuel | [1] | $ 56.5 | $ 58.1
Materials and Supplies |  | 49.0 | 49.1
Raw Materials |  | 2.6 | 2.7
Work in Progress |  | 0.8 | 0.0
Finished Goods |  | 7.4 | 7.5
Reserve for Obsolescence |  | (0.3) | (0.3)
Total Inventories |  | 116.0 | 117.1
Prepayments and Other Current Assets [Abstract] |  |  |
Deferred Fuel Adjustment Clause |  | 12.4 | 10.6
Restricted Cash | [2] | 11.5 | 5.6
Other |  | 17.6 | 19.5
Total Prepayments and Other Current Assets |  | 41.5 | 35.7
Other Non-Current Assets [Abstract] |  |  |
Restricted Cash - Non-Current |  | 8.1 | 8.1
Other Current Liabilites [Abstract] |  |  |
Customer Deposits |  | 14.0 | 15.1
Power Purchase Agreements |  | 23.6 | 23.3
Other |  | 45.2 | 47.7
Total Other Current Liabilities |  | 82.8 | 86.1
Other Non-Current Liabilities [Abstract] |  |  |
Asset Retirement Obligation |  | 136.2 | 131.4
Power Purchase Agreements |  | 131.9 | 138.1
Contingent Consideration | [3] | 36.7 | 36.6
Other |  | 42.3 | 42.9
Total Other Non-Current Liabilities |  | $ 347.1 | $ 349.0
[1] | Fuel consists primarily of coal inventory at Minnesota Power.
[2] | Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and cash pledged as collateral for U.S. Water Services’ standby letters of credit.
[3] | Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 3. Acquisitions and Note 5. Fair Value.)

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